High-frequency electrical communication system utilizing damped oscillations



Feb 22, 194%. W. A. BEATTY 2,

I HIGH-FREQUENCY ELECTRICAL COMMUNICATION SYSTEM UTILIZING DAMPED OSCILLATIONS Filed Jan. 24, 1941 sfshee'ts sheet 1 5 uvmWme Feb. 22, 1949. w. A. BEATTY 2,462,061

HIGH-FREQUENCY ELECTRICAL COMMUNICATION 5 SYSTEM UTILIZING DAMPED OSCILLATIONS Filed Jan. 24, 1941 5 Sheets- Sheet 2 Fig.8. s5 '55 55 H Fig. 9. 59 s9 59 HIGH-FREQUENCY ELECTRICAL COMMUNICATION Feb. 22, 1949. I w BEATTY 2,462,061

SYSTEM UTILIZING DAMPED OSCILLATIONS Filed Jan. 24, 1941 5 Sheets-Sheei 4 Fig. 17.

196. 2707/4/77/05 fi l/1155 500/966 flaw/1am sPc/rrck I16 I17 I38 Fig. 18.

W M W Filed Jan. 24, 1941 Feb. 22, 1949. w. A. BEATTY 2,462,061

HIGH-FREQUENCY ELECTRICAL COMMUNICATION SYSTEM UTILIZING DAMPED OSCILLATIONS 5 Sheets-Sheet 5 H0 Fig. 24.

?atented Feb. 22, 1949 NET D STATS ET OFFICE HIGH-FREQUENCY ELECTRICAL COMMUNI- CATION SYSTEM UTILIZING DAMPED OS- CILLATIQNS Application January 24, 1941, Serial No. 375,814 In Great Britain November 10, 1939 Section 1, Public Law 690, August 8, 1946 Patent expires November 10, 1959 Claims.

In the British Patent No. 511,222, there are described various types of pulses used for methods of pulse modulation at a transmitter, while in copending United States application Ser. No.

313,041, filed January 9, 1940, now Patent No.

2,256,336, issued Sept. 16, 1941, are described methods of generating such pulses. In known pulse modulation systems it has been the practice to obtain intelligence in a receiver by discriminating between the intervals of time oc- 10 struct solid pulses givin the desired intelligence. 20

In cases where single marking pulses coded as S/RL or S/RT are transmitted the suppressed fixed edge marking pulse can be derived by known means and inserted at the receiver so as to reconstruct the corresponding solid pulses giving the desired intelligence. In all these time modulated pulse systems the intelligence obtained at the receiver is a function of the total time occurring between the leading and trailing edges of a train of time modulated pulse systems, and it is necessary to have information regarding the time of commencement and time of finishing of each individual pulse.

According to the present invention, successive damped oscillations are set up at the transmitter or at the receiver, the amplitudes being determined by the amplitudes at successive instants of a sound or like intelligence Wave. The amplitude of each damped oscillation may be determined by the phase of a preceding oscillation at the instant when the damped oscillation is initiated. The oscillations may be generated under the control of a time-modulated pulse train.

The preferred form of the invention utilises pulses which are time modu ated as a function of the amplitude of a sound or like wave in such a manner that if required the desired intelligence can be obtained from one edge only of the time modulated pulses. In known pulse systems it has been the practice to have under modulation conditions both the leading and trailing edges of the recurring pulses variable in time, or to have one series of edges fixed and the other series of edges variable in time. In the system to be described the desired intelligence is obtained from variably recurring edges of time modulated pulse, the time modulation however being a very small proportion of the average time of recurrence of said pulses. In this proposal, the variable edge of the new type of pulse will be referred to as the fluttering edge, while the type of modulation will be referred to as phased pulse modulation, this choice of descriptive terms becoming apparent from the description which follows.

The type of transmission covered by the proposal has the following advantages:

Transmission systems in accordance with the invention can be made extremely economical in power, giving a high signalto-noise ratio; they are very suitable for multiplex transmission and can be readily adapted for secret communication.

The operation of the system is dependent upon transmitting a train of pulses-so time modulated that they have function characteristic of the amplitude of a sound or like wave, said time modulation being characterised in that the total limits of modulation equal a period of time which is equal to the time taken for a fraction preferably less than /2 of a wavelength period of a radio frequency which is comparatively large compared with the pulse frequency; and at a receiver utilising said pulses to control the interruption of a train of oscillations at a frequency equal to said radio frequency, the intelligence received being a function of the amplitude of the damped oscillations at the end of the pulse trains.

The description which follows may be more readily understood by referring to the accompanying drawing in which:

Fig. 1 shows a simple tuned circuit energised by a train of pulses shown in Fig. 2;

Figs. 2, 4, 8 and 18 show various pulse trains required for the system;

Figs. 3, 5, 6, 9, 10, 11, 12 and to 24 show oscillation trains developed in carrying out the invention;

Figs. '7, 13, 14 and 19 show circuit arrangements for carrying out the invention;

Figs. 15 and 16 illustrate cathode ray tubes utilised for generating certain pulses; and

Fig. 1'7 shows a multiplex transmission arrangement.

When a simple tuned circuit is excited by a wave of constant rectangular form certain effects readily to be foreseen are noticed.

Referring to the drawing and more particularly to Fig. 1, a simple tuned circuit 3|, 32 having for instance a natural frequency of 1 megacycle is shock excited by a rectangular pulse 33 Fig. 2.

having a duration of microseconds. The

leading edge 34 of the pulse 33 sets up a train of damped oscillations 35 Fig. 2, the first halfwave of thev oscillatorytrain being shown as a negative cycle. The train of oscillations 35 dies away before the time of occurrence of the trailing edge 36 of the pulse 33, said trailing edge setting up the train of damped oscillations 3? which are similar to the train 35 except that the first half wave of the oscillatory train is shown as a positive cycle; the damped oscillatory trains 35 and 31 in both cases having duration of about 40 microseconds.

If now the circuit 3|, 32 is shock excited by a pulse train as shown in Fig. 4 in which the pulses 38 have a duration of approximately 20 microseconds and are separated by time intervals 39 of approximately the same value, and if arrangements are available for accurately altering the relative durations of the pulses 38 and intervals 39 the following effects are obtainable.

Referring to Fig. the damped wave trains 40 are obtained during the duration of the pulses 33 while the damped wave trains 4| are of different amplitude obtained during the interval periods 39, this relationship between the trains 43 and 4| being dueto the following effects. Considering the first wave train 40 derived from the edge 42 of the pulse 38 the commencement of this train must obviously be similar to the train 35 derived from the edge 34 of the pulse 33. We have shown that the train 31 is the same as the train 35 except that it is reversed in phase, that is to say, its first half-cycle is positive. The trains 40 are derived from the leading edges 42 of pulses 38 while the trains 4| are derived from the trailing edges 43 of the same pulses, these leading and trailing edges being similar to those of the pulse 33. If the edge 43 tends to set up the train of oscillations 4| durin a period of time when the damped train 40 is in phase with the build up tendency for the train of oscillations 4|, it can be seen that this train will now have a greater initial amplitude than that shown for the train 31 If the duration of the pulses 38 are now altered by a slight amount matters can be so arranged that the trains 44, Fig. 6, have such a duration that at the time when the edge 43 tends to set up the train 45 the train 44 is out of phase with the build up tendency of the train 45, the result being that these latter trains have an initial amplitude less than that of the train 31. In practice a change due to pulse duration in the initial amplitude of a train derived from the edge 43 is usually accompanied by an amplitude change in the reverse sense in the train derived from the edges 42 after the first pulse period, since the phasing of the train must modify the next following train thus making the energy content of alternative trains complementary. It can be seen therefore, that taking the two trains 40 and 4| and comparing them with the trains 44 and 45 that practically no change in average amplitude is obtained and these pulse trains are not useful for receiving intelligence in the forms shown in Figs. 5 and 6.

The change of duration for the pulse 38 required to change from train 4| to the train 45 is usually a small fraction of an oscillation period of the shock excited circuit, and in fact when using a shock excited circuit, having a natural frequency of 1 megacycle considerable modulation of the damped train can be obtained with a time modulation of .1 microsecond.

One method of usin the features shown above only 4 microseconds with an interval of 38 micro-' 4. for the purpose of transmitting and receiving in telligence will now be described. In the aforementioned U. S. .Patent..N.o.. 2,256,336,. there are described methods of generating pulses having a time function characteristic of the amplitude of a sound or like wave, one of the types of pulse described in which the leading edges of successive pulses occurat equal time intervals being coded as RL pulses.

The pulses 38 can be considered as a train of RL pulses in which the trailing edges 43 are time modulated so as to be characteristic of the amplitude of a sound wave, the time modulation for maximum to minimum signal amplitude lying within the period of time required to produce the change from train 4| to train 45. Said pulses 38, are, referring to Fig. 7, used to shock excite the circuit 3|, 32 as previously described, the pulses bein applied over the terminals 46. One end of the inductance 3| is connected to the grid 41 of the triode valve 48, having cathode 49 and anode 50. A resistor 5| in the grid cathode circuit bypass condenser 52 and terminals 53 and 54-c0nnected thereto complete the circuit as shown.- The anode voltage of the valve 48 issuch that the valve functions as an anode bend detector under normal operating conditions.- The pulses 38 however, in addition to being utilised for shock exciting the circuit 3|, 32, arealso utilised to.

change the operating characteristics of the valve 48, this being achieved by feeding the pulses 38 between the terminals 53 and 54, terminal 53being positive. The increase in bias due to the pulse causing current to flow in the resistor 5| is sufficient to bias the valve 48 well beyond cut off rendering it ineffective as a detector during the duration of-the pulses 38. Immediately, on the cessation of a pulse 38 the valve is restored to the operating condition and detects the train of oscillations existing during the following'interval 39, and since these trains which will be referred to as interval trains are cyclically varying in their initial amplitudes in accordance with the small percentage time modulation of the pulses 38, the integrated energy of said interval trainsis a function of the amplitude of the sound signal responsible for the time modulationof the pulses 38.

It can be seen from the above descriptionthat the time modulation of the edge 43 is so small that this edge can be regarded as fluttering, while the functioning at the receiver is dependent upon phasing the fluttering edge with a high frequency oscillation, thus explaining the earlier choice of the terms fluttering edge and phased pulse modulation.

Although the waveforms of Figs. 4, 5, 6 can be used for the transmission of intelligence in the way described, they have been described mainly to indicate the development of prepared systems of greater efficiency and simplicity. In practice there are many advantages to be gained by keeping the duration of the shock exciting pulse a relatively small percentage of the intervals between successive pulses. With the simple type of circuit already described it is of advantage to have the pulse duration very short for instance seconds. Such a pulse system and the trains derived therefrom are shown in Figs. 8, 9 and 10;

Referring to Fig. 8, the RL pulse 55 having an average duration of 4 microseconds have leadin edges 56 and trailing edges 51, and are separated by interval periods duration of 36=microseconds.

I58 of an average the trailing edge of the pulse.

tem will be referred to as flutter modulation.

The flutter modulated pulses 55 are utilised to shock excite the simple circuit 3 I 32 as previously explained, and referring to Fig. 9 there is shown a damped train of oscillations 58 due to the excitation from the edge 56 of the pulse 55, followed by another clamped train of oscillations 59, due to the shock excitation caused by the edge 51 occurring at such a time that the build up tendency of the second train is in phase with the damped train 58.

In Fig. 10 there is shown the condition when the flutter modulation of the edge sets up a condition such that the damped train 69 derived from the edge 55 is out of phase with the build up tendency of the train El. The trains 59 and GI both die away prior to the time of occurrence of the next leading edge 56, therefore these trains have no phasing influence on the subsequent trains 58 or 60, these trains being identical under all flutter modulation conditions. Owing to the fact the time modulation of the pulse 55 is very small, the trains derived from the leadin edges of the pulses can be regarded as having substantially constant energy; moreover, owing to the short duration of the pulse the trains 58 and 69 will not have been greatly decreased in amplitude at the time of build up of the pulses 59 and Si, and therefore the amplitude of these interval trains can be greatly influenced by the pulse trains which precede them.

When using an arrangement as described having reference to Figs. 8, 9 and 10, it is not necessary at a receiver to suppress the energy in alternate pulse trains, as one train is practically con stant, the other being variable as a function of the flutter modulation. It can be seen -therefore that in addition to considerable economy in power at the transmitter by having to transmit only a very short pulse, the receiving arrangements can also be simplified.

The arrangements so far described while they permit of the transmission and reception of intelligence, they do not take full advantage of the possibilities of the system. In practical applications it is desirable to arrange at the receiver that the train produced during the duration of the pulse has such a value that it has the maximum effect on the amplitude of the train due to This can be readily achieved by utilising the pulse to drive a regenerative valve circuit into a condition of sustained oscillation and to ensure that the amplitude of this oscillation is such that it can have the maximum eifect due to flutter modulation on the change of amplitude of the trains in the interval periods. Sustained pulse trains 52 during the pulse periods followed by trains 63 of large amplitude are shown in Fig. 11 while in Fig. 12 the interval trains 65 due to flutter modulation have practically no amplitude. By trial methods an amplitude of sustained oscillation t2 and 6 1 can be found such that the interval train can be 100% amplitude modulated by the flutter modulated pulses.

A circuit arrangement shown in Fig. 13 suitable for the production of the wave trains shown in Figs. 11 and 12 will now be described. A triode valve 65 having cathode 61, control grid 58 and anode 69 is connected to a simple regenerative circuit in which the tuned grid circuit has inductance lb and condenser H, and the anode circuit a reaction coil 12. A variable resistor '13 is connected across the tuned grid circuit, the grid condenser I- l grid leak resistor 15 and input terminals It and I! complete the circuit.

The flutter modulated pulse 55 is fed to the terminals I6 and "il, the former being positive, the amplitude of the pulse being sufficient to set the circuit into sustained oscillation. Oscillations quickly build up in the valve circuit reaching a steady value, as shown by the trains 62 and (it, when the anode supply derived from the pulse is rapidly switched off by the trailing edge 51 a damped train of oscillations is set up in the tuned grid circuit, these oscillations being shown as 63 Fig. 11 and st, Fig. 12, their amplitude being dependent upon the phasing of the edge 5'! relative to the sustained oscillation as previously explained. The damping of the trains 63 and 65 is regulated by the variable resistor I3. The trains of oscillations in the circuit 'IIJ, II can be coupled in any well known manner to a suitable detector, giving the desired sound intelligence characteristic of the sound wave responsible for the flutter modulation of the pulses 55.

A circuit suitable for detecting the trains of oscillations in the circuit I0, II of Fig. 13, and at the same time separating the sustained pulse trains 52 for the varying damped trains 63, 55 is shown in Fig. 19.

Varying damped trains are obtained from a train of RL pulses in a similar manner to that described for Fig. 13 by means of valve Ito having anode l lI, control grid I42, and cathode I43, with a bias resistance EM and bias by-pass condenser Hi5, grid leak Hi5, grid condenser M1, and a tuned grid circuit consisting of condenser I48 and inductance Hi9 coupled inductively to a reaction winding H50 in the anode circuit. If RL pulses, Fig. 20, are applied to terminals I5! and the latter being made positive, a sustained oscillation and its accompanying damped train will be formed, Fig. 21, the degree of damping being adjusted by resistance I53. This is then fed into valve I56 which with its associated components forms the first step in separating the continuous train from the damped train.

The mechanism of this second stage, consisting of valve 55% having anode I51, suppressor grid 658, screen grid I53, control grid I60 and cathode Ifii, with anode resistance I52, bias resistance IE3, bias by-pass condenser I66 and grid resistance I55 is as follows. The anode resistance I62 is connected to a steady H. T. supply, and the signal train, Fig.- 21. is fed to the control grid I50 via condenser I54. The operating conditions of the valve I55 are then caused to vary in synchronism with the EL pulses supplied to terminals I5I and I52, by connecting the screen grid I59 to terminal I52, so that, during the duration of the positive pulse I65, Fig. 20, while the sustained pulse train is being generated in valve i ld and its associated circuit, the anode current of valve I 56 is increased, so causing its anode voltage to drop, with the result that the output from valve I56 is as shown in Fig. 22. It should be noted that the residual voltage V1, Fig. 20, between the positive pulses I65 must be such that valve I56 will amplify the damned train. I65, Fig. 21., but will not permit valve I40 to oscillate.

The output from valve I56 is now fed into a leaky grid detector stage consist ng of valve I61 having anode I63, control grid I69, and cathode Im with anode resistance IIl, grid leak I12 and gridcondenser H3 rearranged that theadamped train'l89, Fig. 22, is rectified-while the negative pulse extends past the cut-off point of valve-l6? so that the continuous train portion of the sig nal 199, Fig. 22, is eliminated and the output from valve iii'l is as shown in Fig. 23, section I75 appearing as a positive pulse.

Thispositive pulse appearing between the rectified damped trains can now be cancelled by feeding the signal to the anode o-f'valve l'itl through a condenser H4. Valve Hi5, consisting of anode ll, suppressor grid 118, screen. grid lid, control grid H26 and cathode lili, together with anode resistance 582, cathode resistance 1'83 and cathode by-pass condenser Hi l is made to develop a negative pulse across its anode resistance I82 equal to the signals positive pulse W5, Fig. 23, and in synchronism with it, by feeding the original positive pulse from terminal l 52 to its control grid ltd via resistances 185 and:

' 588, the latter being made variable to facilitate exact adjustment. The output voltage now anpearing at terminals 187 and E88 is as shown in Fig. 24 being the rectified damped train only, as required. a

The applications so far described depend upon thetime modulation of the pulse being such that the actual duration of the pulse is changed, but the system can also be made to operate by slightly changing the time of occurrence of a pulse which: itself remains of constant duration. In the pre-- viously mentioned U. S. Patent No. 2,256,336,

' there are described various types'of marking pulses of relatively short duration but occurring at varying intervals of time, the variations in said time intervals being a time function of the amplitude of a second or like wave. If the time of occurrence of the above type of pulse is varied Within the limits of the flutter modulation already described, such pulses can with special re 'teling pulse, the blocking circuit being released for the pulse duration. This blocking action may be performed in a manner known in the art, for example, by control of the grid bias or plate po tential of an amplifier tube. A variable damping device 82 which may be athermionic valve is coruected across the tuned circuit 89; 8|, the

- characteristics of the device, also under the control of the fluttering pulses being such that dur'-- ing the duration of the pulse the circuit is lightly damped while during the'intervals between pulses the circuit is heavily damped? This can be "achieved utilising the pulse to back bias (i. e. to

apply a negative bias) during the pulse period a thermionic valve connected as a load across the circuit 8!], 8! and to allow the valve to assume a low impedance during the intervals between pulses The circuit 8%. BI is loosely coupled to the tuned circuit 83, 84, which has a variable damping resistor 95 connected across it. The circuit 83, 84 is connected between the'cathode 8B and control grid 81 of the valve 88 which has an anode 89.

1.. 'A reaction .coil 90 is connected between the anode 50 litude modulated receivers.

89 and fl'iB'iJlL-lfll'lilliiil 9 l I The" cathod'e 86 is corinected: to the terminal 92;the condenser flvand riddeak 94. "completing the 'circuit. The coupling between-the coils -fii' and 9B- is such that ien H. T. is applied to the valve '88' the circuit docs'no-toscillate." Theoperation' of the arrangements is as follows.

- assume that the -panes, Fig. Bare-fluttering pulses-,and that they are simultaneously-fed to l-the blocking circuit '59, the clamping device 82 and a e-an" H. supply-to'thevalve 38. A- pulse 35 releases' the-blocking circuit 7-9 allowing iadio -rcquency energy to be fed froiri the -source 78 tOliZ-IGClIGllifibQf 8i; at the sametime the-damp- "=ing'-on the circuit 853, 8!, removedby means of the variable damping device 82, allowing asus- -=tained o'scillationto be built up in the circuits 8i and '83; 84;"the valve-88 serving to' amplify i oscillation, since it"simultaneously received an H. T; supply from the pulse 552*" The oscillations in' the-circuit 33, 34 have a-definite" phase to"the oscillations in the -'supply source "58, which is maintained in a stable conditionboth as regard's'frequencyand phase" drift.

Whezvthe fluttering 'pulse 'ceasestl'iia'time of canon has such a relationship to the "phase of one oscillations in the circuit 33; 8d, as to set up a train of oscillations similar to *those shown at w and-'55 in Figsi Hand 12. Forcorrect operation it is necessary that th-e*"pu1se*55'in theunmodulatedconditionhas such a-phaserelationship with the-oscillations irrthe ci'rcuit'-83;-84 as to setupan interval train which has an energy "content which is the mean of the energy contents 35 of the trainsfia and -65. This correct phase relationship between the pulse-55and the radio frequency oscillations 'can be'obtained by adjustthe coupling between the circuits 80, 8| and t lpor' by the introduction of any known ising device between the supply source! and tne circuit 33, 8d.

'On the cessation'of a pulse 55 the'damping "device'fiii rapidly clamps the oscillations in the circuit Zil, 3i and this circuit therefore has little influence on the damped train set, up in the loosely coupled circuit 83, 84.

The fluttering pulse system has an advantage over the fiuttermodulation 'system'in that the system cannotgive intelligence to ordinary am- The system also "offers a. high degree of secrecy since the amplitude of the flutter of the fluttering pulse or flutter modulation for 100% amplitude modulation must be associated with a. definite frequency for the shock excited circuits in the receiver.

Therefore for reception purposes, in addition to being familiar witlithe general method employed it is also necessary, to'know what particular frequency to use in the shock 'excited circuits.

Owing to the fact that'flutter modulated, or

fluttering pulses occur at substantially equal time intervals, use can be madeoi this fact to reduce at receivers the liability to interference by short duration random signals having an intensity greater than-that of the pulse signal.

"It has been already shown that the actual duration of a pulse is not important for the setting up of the desireddamped trains, for instance pulseszhaving a duration of approxi- 7 mately 4} microseconds can be used with interval periods of 36 microseconds, or pulses of approxir m-atel'y 2 microseconds with intervals of 38 microseconds will give practically'the same final result.

' Assuming: a transmitted pulse 'of approxi- -mately 4' microseconds and interval of approximately 36 microseconds, at the receiver a shorter fluttering pulse for instance, 2 microseconds with intervals of 38 microseconds can be obtained by feeding the pulses through a delay circuit having a delay of approximately 42 microseconds and combining each delayed pulse with the next succeeding undelayed pulse. This method can be extended by employing a second delay circuit having a delay of approximately 82 microseconds, and combining this delayed pulse with the previously derived pulse. Methods of combining delayed and undelayed pulses are described in British Patent No. 528,192.

The delay circuits for the above purpose are well known, but in cases where itis required to accurately adjust the duration of the derived pulse, this can be done by having a Vernier delay arrangement in series with the more usual delay circuits. Such Vernier delay or phasing arrangement can be an electronic device, utilised as described in British Patent No. 519,747.

The utilising of delayed and undelayed pulses at the receiver makes the system practically free from interference occurring at random intervals no matter how intense this interference may be.

In addition to the above expedient, an amplitude limiter may be used in a receiver, matters being so arranged as is usual in pulse transmissions to utilise only the tips of the transmitted pulse, for the purpose of establishing the received intelligence.

The proposals so far described utilise flutter modulated pulses or fluttering pulses in which the time modulation is a very small percentage of the time of recurrence of such pulses. phase pulse system can also be applied for the purpose if discriminating between rectangular pulses of constant amplitude but of widely differing durations, as for instance line and frame pulses in television synchronising systems.

In a known television system line pulses have a duration of ten microseconds while the frame pulses comprise a train of pulses having a duration of 40 microseconds. If such pulse are utilised to shock excite a tuned circuit for instance according to the type described having reference to Fig. 13, the natural frequency of the shock excited circuit can be such that the train of pulses set up at the end of the 40 microsecond frame pulse is of the type shown in Fig. 11, while the train set up at the end of the 10 microsecond line pulse is of a smaller amplitude for instance, as shown in Fig. 12.

The fact that any desired phasing between a high frequency signal and pulses of difierent du rations may be obtained can be readily appreciated by taking a simple example. If we assume that during the period of a line pulse 1. e. 10 microseconds the frequency of the shock excited circuit is such that an integral number of oscillations plus a half oscillation occur, thenduring the p riod of a frame pulse i. e. 40 microseconds, an integral number of oscillations take place, therefore the trailing edge of the line pulse can be regarded as 180% out of phase while the trailing edge of the frame pulse, and the damped trains following these respective pulses will have different amplitudes.

It should therefore be obvious that by suitably choosing the frequenc of the shock excited circuit the amplitude of the trains of oscillations, at the end of one pulse can be made larger than the other. and for the purpose of obtaining accurate frame synchronisation matters are so arranged that the damped train at the end of the .10 frame pulse has the larger amplitude. The damped trains are now fed to a detector and amplitude filter so arranged that an output can only be obtained from trains of larger amplitude, said output being utilised for frame synchronising.

This method of pulse discrimination can also be applied in cases where discrimination isre-' quired between one or more pulses of a pulse train having pulses which have three or more different durations.

For instance, if it is required to use the shortest and-the longest pulse of a train of three pulses of diiferent durations, for the purpose of synchronising apparatus, a suitable frequency can be found such that the amplitude of the trains at the end ofthe shortest and longest pulses is greater than the amplitude at the end of the other pulse, and as described above the shortest and longest pulses can be used for synchronising.

It can therefore be seen that this method of pulse discrimination is much more flexible than known systems.

The system can also be applied to pulses in which one edge is flutter modulated giving one kind of intelligence while the other edge is time modulated as described in the afore-mentioned U. S. Patent No. 2,256,336 It is therefore possible for one kindof intelligence to be conveyed by the fluttermodulation of one edge and for another kind of intelligence to be conveyed by the ordinary time modulation, and if the latter modulation is comparatively large it will entirely mask the flutter modulation intelligence in ordinary amplitude modulated receivers which however, will give intelligence from the ordinary time modulated pulse. For instance, if the pulses coded as-RT in U. S. Patent No. 2,256,336, that is to say rectangular pulses having their trailing edges occurring at in such a manner that the trailing edge is caused to flutter two kinds of intelligence can be simultaneously transmitted by'one pulse. .This'type of pulse transmission will. for the purposes of convenience be coded as RF'T (rectangular with fluttering trailing edge the leading edge being variable); the corresponding" BL pulse being coded as RFL.

Such a system may be utilised for stereophonic transmission, the separate sound waves constituting the two kinds of intelligence; 7 H

A method of generating such a double-intelligence pulse system will now be described. Referring to Fig. 11 of the drawing accompanying U. S. Patent No. 2,256,336, there is'shown a target arrangement of the cathode ray tube which is suitable for generating pulses of constant duration but having varying times of occurrence, such pulses being utilised as marking pulses indicating the leading edges of solid pulses. Using similar method to those described in the above-mentioned U. S. patent, fluttering pulses can be generated, the flutter being characteristic of the amplitude of a sound wave. Referring to Fig. 15 of the drawing accompanying this specification there is shown a target I33 of a cathode ray tube arrangement i9 4 similar to that described having reference to the Fig. 11 of U. S. Patent No. 2,256,336. The target plate H93 is shown as a straight edged plate disposed nearly at right angles to the direction I35 of scanning. This arrangement oftarget plate ensures that for large amplitudes of transverse 1- modulation of the beam due to the amplitude of the sound wave. a relatively Small timemodulation of the desired pulses obtained- The same linear scanning source. which causes the scanning of the target I 93:..is: also used to scan the target 96 of a similar cathode/ray tube arrangement 9'5 in the direction 98. The: beam intthis second tube being transversely modulated under the control of the amplitude :of another sound wave. We now have a train of pulsesoi constant duration occurring at considerably varying time periods. obtaine.d':from the'tube 97, while we have ancthen fluttering train of, Pulses ohtainedirom the tubeflfi. These twomarking pulse. trains can in anyrknown' manner be utilised to. establish :solid'type pulses which can be transmitted; or alternatively the pulses" can be transmitted as a :double pulse system, and the corresponding solid pulses reconstituted at the receiver.

'Whichever'of these methods is adopted the resulting RFT pulses acan be :utilised to give'in ordinary amplitude modulated receivers inte1li-- gen'ce corresponding to the sound wave causing the time modulation Of the; ipulsesderived from the -target Sipwhile the fluttering edge of the BET pulsesxcanbetutilised torgive with a. special receiver intelligence corresponding to'the .sound wave causing the fluttering of the pulses obtained'irom the target I93. 'FIhe'receiver required fior' obtaining the intelligence. :iromthe fluttering edge aof'.the'RPTpulseiis-similar to that/described I havingreference to sPig. '14., except 1 that .the

The time modulation of the RF-Ipulses'should beskept'within such limits-that the variable edge of one pulse doesnotapproachtoo close tothe fluttering edge of theprecedi-ng. pulse, since sufiicient time mustbe allowed for the interval traimof oscillations to idle away before thecommencement of the-succeeding pulse. This re quirement tosome extent limits the :depth :of timemodula'tion which can be. given to.- the pulse.

"The. RFTQpulse above: described'although ca pable of 'simultaneouslygiving two: kinds of intelligence, is to some. extentv subject to weak. cross time modulation of the flutter intelligence on .the time modulation intelligence. In practicev this is not serious as the flutter timemodulation is generally very much. less; than .thezmain time modulation This tendency to :cross. time: modulation can however, b .obviated by arranging that a train'of. RT. pulses having a time function characteristic of .one kind-moi intelligence, are fluttered so that theyihave. a flutter characteristic another kind oi intelligence These Pulses. will for the purpose of convenience be coded asMRIT-l-F (rectangular pulses with trailing edge normally fixed and leading edge variable,. the leading and trailing edges being fluttered .modulatedin phase bysimilar amounts). I

'A method of generatlng thgRT-l-F pulses. will now-be described, this being, a} modiflcationof. the methodused viorgenerating RFI. pulses. :Asdescribed-for the generationloiRF'I'pulses ail-utter modulated pulse train isv derived from the. plate I93, and these pulses are .utilised tosynchronise a-second timebase giving a linear waveform suitable for scanning the target. 516.. The. flutter time modulation can .be less. than. 1% ,oi-the average timeof recurrence .of the. pulses. and therefore no-difliculty is experienced in maintaining the.

12' second time'base. in :synchronism with the fluttering pulses; Thev effect of using a second time basetis .toadvance-or retard the normal time of occurrence of the variable edge marking pulses in accordance with the flutter modulation of the leading :edgecmarking pulses. It will be seen that the type .of pulse. established by this method has the leading edge occurring at nearly equal intervals while the trailing :edge occurs at widely varying intervals, whereas at the receiver the opposite condition should hold. The' type of pulse which has: beengenerated will for the purpose of convenience be coded as RL+F and this can atthe transmitter or receiver be readily changed to an RT-l-Fpulseby passing the pulse through a single stage valve amplifying stage which also changes thephase of the pulse. Such changing of pulse phase does not afiect the intelligence which is obtained from an amplitude modulated receiver but has, however, theefiect of changing the flutter control edge from being a leading edge to a trailing edge. The RT+F pulses are utilised in a similar manner for the purpose of shock exciting a suitable receiver circuitas has been described for the BET pulses. This latter method of transmission has the advantage that bothkinds of intel' ligence can be transmitted without the possibility of mutual interference, and if the existence of thelflutter modulation is not suspected, the intelligence due to the flutter modulation will not be detected.

Owing to the. fact that only one train of pulses having a duration which is relatively small compared with the interval between successive'pulse 5 and which do not vary greatly in their time of recurrence, is required for the. transmission of intelligence, the system is. particularly suitable for the multiplex transmission of intelligence over onechannel, since .byselective allocation of time intervals to different channels difierent flutter modulating plates.

modulated,.or fluttering pulses can be allocated to said channels. I

One method of performing multiplex transmission will be more readily understood by referring to .Figs. 16, 1'7 and 18. A cathode ray tube 99 Fig. 16, has8. similar pairs of target plates the plates of each pair being arranged one behind the other. The plate [00 is larger than the plate I'QI and is placed behind itas. shown.' PlatesIOU, H12, I04, I06, I08, III], H2 and I are utilised as circuit switching plates, while the smaller plates I'!) I I93, I05, IIl'I,v me, III and H3 are. used for the purpose of generating flutter modulated pulses. The

plates as shown have parallel sides, this particular shape being taken for exemplary purposes only. Electrostatic deflecting means are available in the cathoderray tube, and an electron beam follows a circular path I31 across all the switching and The alternating current source H6, Fig. 17, supplies current at a frequency equal to the desired pulse frequency to an amplitude modulating unit I I1 and a phase splitting .unit I38, whence the alternating current is fed in phase quadrature to the deflecting plates of, the cathoderay tube. giving the. trace I31 al.- ready mentioned. Y

.One of therblolcking circuits. ll8l25 is interposed between each source of intelligence Iii-43.3, a separate blocking circuit being allocated to each intelligence source. Normally the several1 blocking, circuits operate co-prevent signals from the various intelligence sources from reaching the modulatingunit I I1.

.Considering'the. electron beam in the tube 99 just before it strikes the plate 10.0., the operation 13 is -as follows. The beam is not resting on any plate and inthis'condition all the blocking circuits are operative. Immediately the beam strikesplate I09 a pulse is derived therefrom and this pulsecan be utilised to trigger a double stability circuit of known type setting up the pulse I34, Fig. 18. The pulse I34 serves to so bias the blocking circuit H8 that it becomes inoperative and allows the signal intelligence from source IZ-a to pass to the modulating unit II'I, thereby changing the amplitude of the alternating current from the source H in accordance with the amplitude of the signal from the source I26. Assuming that the change in amplitude is such that the signal fed in phase quadrature from the unit 4 giving a pulse having a duration which is a time L,

function of the amplitude of the signal from the source I26. Immediately the beam leaves the plate till it once more strikes plate IDEI giving another pulse serving to trigger the double stability circuit in the reverse direction and thus discontinuing the pulse I34, whereupon the bias is removed from the blocking circuit H8 thereby making this circuit operative once more and cutting off the signal intelligence from the source I26.

. The electron beam continues on its travel and after a very short time period strikes the switching plate I92 setting up via another double stability circuit the pulse !35, which serves to unblock the circuit H9, allowing the signal from the source I2! to pass to the modulating unit I ll, modifying the diameter of the trace I31 in such a manner that the pulse derived from plate N33 has a time function of the amplitude of the signal from source I 21. The beam now once more strikes plate H32 discontinuing pulse I35. The other intelligence channels operate in a similar manner, the pulse 36 for the pair of plates H t and I l5 only being shown in Fig. 18.

-The shape of the individual signalling plates mi, :63, etc. maybe modified so that any desired lawis obtained. There are advantages in making each signal plate follow. a different modulation law to the other, as by this means dif ferent frequencies for the shock excited circuits can be chosen at a receiving point, thus reducing the possibility of mutual interference between the damped trains generated in a number of receivers located close to one another.

At the receiving end a similar tube to the tube 98 can be provided, having eight switching plates disposed in a similar manner to the switching plates in tube 99, and a source of supply of the same frequency as that of I can be utilised to set up a circular trace giving switching pulses from the various plates. The current giving this circular trace can be suitably phased by any known means so that the receiver switching pulses synchronise with the appropriate trans mission channel. The pulses generated at the receiver are utilised to cyclically allocate in any known -.manner the various flutter modulated pulsestodifferent intelligence channels thus allowing of multiplex operation.

If desired synchronising of the transmitter and i4 receiver pulse frequency can be achieved by utilising one of the communication channels to transmit a frequency which is submultiple of the frequency in the source H5. This signal can at the re iver be put through a suitable frequency multiplier giving the desired controlling fre quency at the receiver.

Owing to the fact that only a very small change in theduration of the pulse is required for the transmission of intelligence, the system can be readily applied to signalling systems which already utilise pulses for other purposes, such systems being for instance television systems.

In pending United States application Ser. No. 325,954, filed March 26, 1940 now Patent No. 2,295,023, issued Sept. 8, 1942, it is proposed that the line or frame synchronising pulses of a television signal to be time modulated to convey additional information such as brightness level, frame sequence for colour or stereoscopic television, or degree of volume compression of an associated. signal.

It is now proposed that flutter modulated pulses be utilised to achieve similar effects. Methodsof applying flutter modulated pulses for such purposes should now be obvious having regard to the foregoing and also having regard to said U. S. Patent No. 2,295,923. In addition to giving the types of additional intelligence above mentioned,

it is possible to effectively use flutter modulated pulses as combined line synchronising pulses, and sound intelligence for the accompanying vision signal. In such cases it is necessary to have the leading edge of the synchronising pulse occurring at equal intervals of time, while the trailing edge is flutter modulated as already explained. If desired certain of the pulses occurring in a frame period can also be flutter modulated so as to also give sound intelligence during this period.

The phase pulse modulation system above described has many advantages when it is desired to communicate over distances in which the time taken for signals to travel from one point to another remains reasonably constant relative to the frequency of the shock excited cl"cuit in use at the receiver. The system however, is not suitable for transmitting over distances which due to ionospheric or atmospheric reflection or refraction change to a great extent the length of the path followed by signals between a transmitt r and receiver. It will therefore be seen that the most useful applications of the system are for communicating on ultra short waves within distances governed by the limitations of such short wave transmissions. Ultra short waves are necessary so as to easily obtain the necessary Wide bands required for the transmission and reception of the pulses.

The system can also be readily applied to cable systems provided the cable can accommodate the wide band of frequencies required for the transmission.

What is claimed is:

' 1. An electrical system for the transmission of intelligence comprising means for producing a succession of damped oscillations and control means comprising means responsive to the phase of a preceding oscillation at the instant when the damped oscillation is initiated for controlling the amplitude of successive ones of said oscillations in accordance with the amplitudes at successive instants of a sound or like intelligence signal wave.

2. A system according to claim 1, further comprising means for applying a time modulated pulse 175 train to control-said means for producing'a succession: of oscillations.

3; Anrelectricalsystem for the transmission of intelligence, comprising means for producing a succession of damped oscillations, control means for-controlling the amplitude of successive ones of said oscillations in accordance with the amplitudesat successive instants of a sound or like intelligence signal Wave, means for applying a time modulated pulse train to control said means for producing a succession of oscillations, said pulse train comprising pulses of a frequency substantially greater than the hi hest signal Wave frequency, and means for variably controlling the pulse duration in accordance with signal Wave amplitude and over a range which is comparable with the period of said damped oscillations.

4. An electrical system for the transmission of intelligence, comprising means for producing a succession of damped oscillations, control means for controlling the amplitude of successive ones of said oscillations in accordance with the amplitudes at successive instants of a sound or like intelligence signal wave, means for applying a time modulated pulse train to control said means for producing a succession of oscillations, said pulse train comprising pulses of a frequency substantially greater than the highest signal Wave frequency, and means for variably controlling the pulse duration inaccordance with signal wave amplitude and over a range which is comparable with the period of said damped oscillations, the duration of said pulses being small with respect to their time spacing.

5. A transmitter, comprising means for gener-' ating a train of time-modulated pulses of highfrequency, means for varying the duration of saidpulses in accordance with the amplitude of a signal wave, and over a range less than the period corresponding to the fundamental pulse frequency means for producing a succession of damped oscillations under control of said time modulated pulses, and meansfor limiting the amplitude of, successive ones of said oscillations in accordance with the amplitudes. of successive ing high with respect to the fundamental pulse frequency of said pulse train and the damping being such that the oscillations have substantially died away at each instant of arrival of a pulse whatever the duration of the previous pulse.

7. An electrical system for the transmission of intelligence, comprising means for producing a succession of damped oscillations, control means for controlling the amplitude of successive ones of said oscillations in accordance with the amplitudes at successive instants of a sound or like intelligence signal wave, means for applying a train of pulses to control said means for producinga succession of oscillations and means for variably controlling the spacing of one edge of successive ones of the pulses of said train in time in accordance with the amplitude of a sound or like intellige'nce wave.

8. An electrical system for the transmission of intelligence, comprising means for producing a succession of damped oscillations, control means for controlling theamplitude of successive ones of 5 said oscillations in accordance-with the amplitudesat successive instants of a sound or like in.-.

telligence signal wave, means for applying a time modulated pulse train to control'said means for producing a succession of oscillations, means for varying the spacing-of leading edges of the pulses of said train from equally spaced time intervals in accordance with the amplitude of one signal,

1 and means for varying the spacing of trailing edges of said pulses from equally spaced time lntervals inv accordance with the amplitude of' a. second signal wave.

9. An electrical system for the transmission of intelligence, comprising means for producing a.

successionof damped oscillations, control means for controlling the amplitude of successive ones of ofsaid train from equally spaced time intervals in accordance with the amplitude of one signal, and means for varying the spacing of trailing edges of said pulses from equally spaced time intervals in accordance withthe amplitude of a sec-, ond signal wave, the time intervals being determined by one set of'edges or marking pulses varying over a range comparable with the interval between the equally spaced time instants and'the time intervals determined by the other set of edges or marking pulses varying over a relativelysmall range.

10. A multiplex system for the-transmission of intelligence, comprising means for producing a succession of damped oscillations, control means for controlling the amplitude of successive ones of said oscillations in accordance with the amplitudes at successive instances of a number of intelligence signal waves, means for applying a time modulated pulse train to-control said means for producing a succession of damped oscillations, and means for variably controlling thep'ulse duration in accordance with the signal wave amplitudes and over a range which is comparable with the period of said damped oscillations, said last mentioned means comprising means for deflecting an electron beam in a circular trace by high frequency alternating currents in quadrature, a plurality of target structures arranged in the path of said beam to be cyclically scanned by the beam, said target structures being designed to present difierent traversal areas at different radial displacements, means for controlling the amplitude of the deflection of said beam in accordance with each of said number of si nal waves. over successive periods determined by the incidence of the beam on successive target structures, and means for collecting from the target structures the determined current pulses of varying duration determined b the momentary radius of the trace.

I WILLIAM ARNOLD BEAT'I'Y.

REFERENCES orrnn The following references are'of record in the file of this patent:

UNITED STATES. PATENTS 

